Medical procedures are available for treatment of a variety of cardiovascular maladies, such as for example cardiac arrhythmias, atrial fibrillation, and other irregularities in the transmission of electrical impulses through the heart. As an alternative to open-heart surgery, many medical procedures are performed using minimally invasive surgical techniques, where one or more slender implements are inserted through one or more small incisions into a patient's body. Such procedures may involve the use of catheters having multiple sensors, electrodes, cryogenic chambers, or other measurement and treatment components to treat the diseased area of the heart, vasculature, or other tissue. Catheter-based devices are desirable for various medical and surgical applications because they are relatively non-invasive and allow for precise treatment of localized discrete tissues that are otherwise inaccessible. Catheters may be easily inserted and navigated through the blood vessels and arteries, allowing non-invasive access to areas of the body with relatively little trauma. Typically, such minimally invasive and intravascular devices are routed through a femoral artery or other passageway into the heart under guided fluoroscopy or other imaging techniques.
One such example of a minimally-invasive treatment modality involves the treatment of cardiac arrhythmias or irregular heartbeats in which physicians employ specialized ablation catheters to gain access to interior regions of a patient's body. Such catheters may include an ablation tip or other ablating elements to create lesions or other anatomical effects that physiologically alter the ablated tissue without removal thereof, disrupting or blocking electrical pathways through the targeted tissue. In the treatment of cardiac arrhythmias, a specific area of cardiac tissue having aberrant electrically conductive pathways with erratic electrical impulses is typically initially localized. An ablation procedure may involve creating one or more lesions in order to electrically isolate tissue believed to be the source of an arrhythmia. During the course of such a procedure, a physician may diagnose aberrant tissue and cryogenically destroy it. Cryotreatment or cryogenic ablation entails creating cold temperatures at specific regions of the body or contacting tissue with cold treatment devices. In particular, cryoablation involves transferring heat from the targeted tissue to the cryogenic element, thus cooling and/or ablating the tissue.
Cryogenic treatments may involve fluids with low operating temperatures, or cryogens, with the use of catheter-based devices employing the flow of cryogenic working fluids to selectively freeze, or “cold-treat,” targeted tissues within the body. A cryogenic device uses the energy transfer derived from thermodynamic changes occurring in the flow of a refrigerant through the device. This energy transfer is then utilized to create a net transfer of heat flow from the target tissue to the device, typically achieved by cooling a portion of the device to very low temperature through conductive and convective heat transfer between the refrigerant and target tissue. The quality and magnitude of heat transfer is regulated by device configuration and control of the refrigerant flow regime within the device. While cardiac arrhythmias present one example, such “cold energy” can be safely and effectively used to treat a host of medical conditions by creating endothermic heat transfer from a surgical tool relative to a region of tissue, so as to induce localized hypothermia of varying severity.
To provide shorter treatment durations and increased efficacy for the particular treatment provided, it is desirable to optimize the heat transfer between the specific tissue to be treated and the cryogenic element or device. In other words, heat transfer from any tissue other than that selected for treatment, such as blood or other body fluids in or passing through the vicinity of the cryogenic element for example, should be minimized or avoided. Such thermal exchange with tissues or fluids other than that targeted for treatment can reduce the thermal exchange with the targeted tissue and also require additional “cooling power” or refrigerant flow to the cryogenic device in order to complete the desired treatment. Accordingly, heat transfer with any thermal load other than the tissue to be treated should be reduced or prevented. It would be desirable to provide an apparatus and method of increasing the efficiency of heat transfer during cryogenic treatments between the tissue selected for treatment and the cryogenic element used to treat the tissue.